There has been known a Pulse Width Modulation (PWM) electric power control method as one of an electric power control method that converts a DC power into an AC power and applies the AC power to a three-phase AC motor.
The general PWM electric power control method sequentially measures a current supplied to a motor at timing at which a carrier wave becomes the maximum or the minimum and obtains a duty command value according to the measured current and a request torque to the motor. The method compares the duty command value with the carrier wave in magnitude and operates on/off of switching elements in an inverter on the basis of the comparison result. This controls a pulse width of an applied voltage to the motor, supplying a desired electric power to the motor.
A semiconductor chip achieves a process regarding the PWM electric power control method. A temperature range in which the semiconductor chip is stably operable is determined in many cases. Therefore, in the case where a temperature of the semiconductor chip itself becomes high or a similar case, if the semiconductor chip generates heat caused by the operation of the switching elements, the semiconductor chip possibly exceeds the temperature range.
Therefore, a technique disclosed in JP2009-100599A changes a frequency of a carrier wave low to lengthen an operation period during which the operation of the switching elements is possibly performed. This configuration lowers the operation frequency of the switching elements, thereby reducing the heat generation by the semiconductor chip.